Multi-Scale Influences of Climate on upper treeline dynamics along a latitudinal gradient in the Rocky Mountains, USA.

Abstract

The Rocky Mountains have experienced broad-scale increases in temperature during the 20th century. Both paleoecological and contemporary research suggest that upper treeline ecotones are sensitive to temperature fluctuations, yet questions remain as to how the complex interactions between temperature, precipitation, and spatial patterns of tree establishment act to ultimately facilitate or limit treeline advance.The inherent complexities associated with these interactions portend the likelihood of crossing certain bioclimatic thresholds, particularly when considering that they are operating within the context of global climate change. Moreover, the relative importance of these variables is likely to change according to slope aspect and by spatial scale of analysis. My dissertation research uses a multi-scale approach, ranging from contrasting south- and north-facing slopes at a local scale to a regional scale along a latitudinal gradient in the Rocky Mountains to ascertain the influence of temperature and precipitation on upper treeline dynamics. Specifically, this research addresses whether temperature is the predominant influence on tree establishment at upper treeline and eventual treeline advance, and how this varies by spatial scale in the Rocky Mountains (ca. 35°N-44°N). From a regional perspective, widespread tree establishment is occurring at upper treeline, and has almost exclusively been initiated since ca. 1950. Similar region-wide patterns of tree establishment at upper treeline across various environmental gradients suggest the importance of broad scale climate inputs in driving upper treeline ecotonal dynamics. Perhaps most compellingly, random spatial patterns of tree establishment have the only significant correlations with temperature, suggesting that they are more directly influenced by climatic inputs compared to clustered spatial patterns. Clustered spatial patterns initially form in response to harsh abiotic conditions, but then become more dependent on positive feedback interactions with neighboring vegetation rather than changes in abiotic conditions. Thus, the conclusions of this multi-scale research suggest that upper treeline ecotones in the Rocky Mountains are indeed sensitive to climate and temperature appears to be the most influential climatic component driving these changes. However, the relative influence of climate on treeline dynamics can vary considerably depending on slope aspect, spatial pattern of tree establishment, mountain range, and location relative to 40°N.